omission

Short definition from Wiktionary

Noun

1. 1
   The act of omitting.
2. 2
   The act of neglecting to perform an action one has an obligation to do.
3. 3
   An instance of those acts, or the thing left out thereby; something deleted or left out.
4. 4
   Something not done or neglected.
5. 5
   The shortening of a word or phrase, using an apostrophe ( ' ) to replace the missing letters, often used to approximate the sound of speech or a specific dialect.